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Lesser Cornstalk Borer

Lesser Cornstalk Borer (LCB), Elasmopalpus lignosellus, has become a serious pest on sand soils and an occasional pest on organic soils.

Adult lesser cornstalk borers are small, slender moths about 1/2 to 5/8 in. long. They are easily disturbed by walking through the field, but these quick fliers usually move no more than 10 feet at a time. They are most active at dawn and dusk. Females are covered with mostly gray to brown and reddish shiny scales. The base color of males is much lighter, ranging from pale yellow to medium brown. Wings of males are bordered with darker brown scales and a small spot is often visible on each wing near the center of the back when the wings are folded. The base color of young larvae is white to creamy yellow. Reddish to brown patterns adorn each thoracic segment, except for the first, which is covered with a broad black shield. Older larvae develop green to turquoise blue color between the darker patterns, particularly between the head and thoracic segments.

Adults deposit their shingle-like, translucent eggs on young shoots near the soil. Larvae emerge from eggs in 3 to 18 days and enter the soil to burrow into soft young tissue usually within 3/8 in. of the soil surface. They feed on tillers and older shoots from within tunnels lined with silk and usually bore-out tissue within 1 inch above or below the soil line. The soil-covered tunnels are often found attached to the entry wounds and serve as an important diagnostic trait to separate their damage from that of wireworms, which do not produce such tunnels. Development is highly dependent on soil temperature. The larvae complete development in 17 to 42 days and can kill several young shoots before pupating in the soil.

Damage to meristematic tissue presents itself as dead young tillers and older shoots with dead youngest leaves (i.e., dead heart). Shoots with dead hearts usually produce additional tillers, so the plants can potentially compensate for this type of early damage. Evidence of feeding above the meristem later becomes visible as rows of holes on the two to three leaves present within the whorl when it was attacked. Frequently, the tips of these leaves break off at the row of holes. Fields with a high frequency of LCB damage may appear to have been mechanically mowed. Susceptibility to damage generally decreases after the shoots reach 1 foot in length.

E. lignosellus is also an important pest of beans, corn, peanuts, and pepper. They feed on other grasses and are often found in association with nutsedges which also exhibit the dead tiller and dead heart symptoms. Weedy fields and those bordered by other LCB host plants may experience prolonged activity associated with adult emergence from these reservoirs.

As with most insect soil-pests, there is little opportunity to control the damaging stages with post-emergence pesticides. While a pesticide formulation is registered for use against this pest, insecticidal control is difficult to achieve because larvae live within the plant tissue and are protected by their silken burrows. Applications in association with cultivation have achieved some limited success. Pesticides applied to the plant tissue or banded over the rows and mixed or covered lightly with soil during cultivation place these toxins in the path of the larvae as they move up the soil profile to reestablish feeding sites within 3/8 inch of the soil surface.

Application of pesticides with enough water to wet the top 3/8 inch of soil around the plants will also place them where they will have the greatest effect against LCB larvae. Adults are sensitive to broadcast insecticide applications and treatments should be timed to coincide with their most active periods.
 

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